580 VALVE OF V1EUSSENS. 



Sylvius or iter e tertio ad quartum ventriculum, is extended up 

 underneath the tubercula quadrrgemina to the third ventricle, 

 which will be found between, and extending a little above the 

 crura cerebri. Among the white fibres constituting the pons, there 

 is likewise intermixed a considerable quantity of cineritious matter. 



The Valvl of Vieussens, 



Arises from the central or fundamental portion of the cere- 

 bellum and is extended upwards in the middle line to the lower 

 two bodies of the tubercula quadrigemina the testes. It is 

 attached on each side to the processus e cerebella ad testis, form- 

 ing as it were a portion of the pons, and part of its fibres passes 

 under the tubercles to reach the thalami nervorum opticorum. 

 It has been confounded in description with the processus cere- 

 belli ad testes ; it ought rather to be called the processus cerebelU 

 ad cerebrum. It covers in its course the top and sides of the 

 fourth ventricle, is formed of a thin medullary lamen on its outer 

 or posterior face and of a delicate cineritious layer on its inner. 

 It received improperly the name of valve, from Vieussens ; from 

 the appearance it presented when its upper end was cut off, by 

 the system of horizontal slicing, with which this anatomist studi- 

 ed the brain ; its detached end there seeming to face down like 

 a valve to shut off the communication between the third and 

 fourth ventricle. The cineritious matter is thickest at the middle 

 portion of the valve, where its structure is least resisting, and 

 from whence arises the fourth pair of nerves. 



The Crura Cerebri 



Formed in the manner already described, are cylindrical, 

 and closely approximated at their origin from the anterior and 

 upper part of the pons. They are about six lines long, and six 

 lines thick, and form the column of fibres, from which the hemis- 

 pheres of the cerebrum are developed. As they pass up they 

 recede from each other, in order to approach the middle of each 

 hemisphere, where they terminate in the corpora striata and 

 thalami nervorum opticorum. They flatten themselves as 

 they recede from each other, and leave necessarily a cavity 

 between them, which forms a part of the third ventricle. The 



